Luminescent materials, which underwent almost 100 years' research and development, are currently indispensable in many important applications including fluorescent lighting, display devices, X-ray imaging, scintillators, and biological imaging [Adv. Funct. Mater. 13: 511; Yen, W. M., Weber, M. J. Inorganic Phosphors: Compositions, Preparation and Optical Properties. 2003, CRC Press LLC]. The luminescent materials used in these applications are generally in the form of powders. Recently, a series of oxide and nitride luminescent materials, such as ZnO, SnO2 and GaN, were made into one-dimensional (1-D) nanowires and nanobelts that can be used as the building blocks for miniaturized nanophotonic circuits [Science 305: 1269]. Such nanophotonic circuits have the functions of light creation, routing and detection, laying the ground for the fabrication of highly integrated light-based devices such as optical computers. Due to the limited optical performance of ZnO, SnO2 and GaN (such as limited luminescent colors and defect-related emission), however, further development of nanowires circuitry needs new types of luminescent nanowires that should have rich luminescent colors and emit characteristic light. Rare-earth (RE)-activated phosphors with diversiform luminescence apparently meets this material need.
RE-activated phosphors are one of the most important families of luminescent materials. In RE-activated phosphors, the RE ions are usually doped into the hosts in either trivalent (RE3+) or divalent (RE2+) states. Most of the doped RE3+ ions have characteristic atomic-like emission spectra, which are attributed to the 4fn→4fn intraconfigurational transitions, due to the well-shielded 4f shell. The RE2+-activated phosphors, in contrast, typically exhibit broad emission bands, which are generally attributed to the parity-allowed 4fn-15d→4fn interconfigurational transitions whose wavelengths depend strongly on the host lattice.
RE2+-activated phosphors, particularly Eu2+-activated phosphors, are receiving increasing attention for their tunable band-like emission and broad excitation range, as well as their many important practical applications. For example, the emissions from Eu2+ ions in different hosts can be tuned from near-UV to red, while the excitation can be extended from blue light to even the X-ray region [Res. Rep. 23: 201]. The tunable and broad emission and excitation bands of the Eu2+-activated phosphors could fill up the spectral gaps in the emission spectrum of current white phosphor-converted LEDs (pc-LEDs) to improve their color quality for general illumination [Proc. SPIE 3938: 30]. The defect-related charge trapping phenomenon followed by normal 4f65d→4f7 transitions in some Eu2+-activated phosphors has led to such important applications as information storage, long persistent luminescence, electroluminescence, and high-energy radiation detection. Besides the normal 4f65d→4f7 transition, some Eu2+-doped alkaline earth compounds also show an extremely broad and red-shifted anomalous emission band originated from a impurity-trapped exciton (ITE) state, which is constructed by a hole on the impurity and a trapped conduction electron on the nearby lattice sites [Phys. Rev. B 32: 8465].